Method of producing steel strip

ABSTRACT

Steel strips and methods for producing steel strips are provided. In an illustrated embodiment, a method includes continuously casting molten low carbon steel into a strip and hot rolling the cast strip within a temperature range such that the strip passes through the Ar 3  transformation temperature from austenite to ferrite under strain during rolling, thereby producing a strip having a yield strength above 400 Mpa and an elongation in excess of 30%. The cast steel strip has desired microstructures.

[0001] This application claims priority to Australian Patent Application No. PR0480, filed Sep. 29, 2000.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] The present invention relates to a method of producing steel strip.

[0003] In particular, the present invention relates to producing steel strip in a continuous strip caster.

[0004] The term “strip” as used in the specification is to be understood to mean a product of 5 mm thickness or less.

[0005] The applicant has carried out extensive research and development work in the field of casting steel strip in a continuous strip caster in the form of a twin roll caster.

[0006] In general terms, casting steel strip continuously in a twin roll caster involves introducing molten steel between a pair of contra-rotated horizontal casting rolls which are internally water cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip delivered downwardly from the nip between the rolls. The term “nip” is used to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed. The casting of steel strip in twin roll casters of this kind is for example described in U.S. Pat. Nos. 5,184,668, 5,277,243 and 5,934,359.

[0007] Hot rolling continuously cast steel strip to achieve a thickness reduction of 30-40% as the strip temperature passes through the austenite to ferrite transformation temperature (commonly called the Ar₃) can produce a microstructure that includes a mixture of ultrafine polygonal ferrite grains and fine ferrite grains that provide the cast strip with superior mechanical properties without the use of micro alloys such as Ti, Nb in the steel. The Ar₃ temperature will vary with steel composition and is the temperature where the austenite to ferrite transformation commences.

[0008] According to the present disclosure there is provided a method of producing steel strip having a yield strength of at least 400 MPa and a total elongation of at least 30% which includes the steps of:

[0009] (a) continuously casting molten steel into a strip;

[0010] (b) hot rolling the cast strip within a temperature range such that the strip passes through the austenite to ferrite transformation under strain during rolling to produce a strip having a yield strength above 400 Mpa and an elongation in excess of 30%.

[0011] In one embodiment, the steel strip cast in step (a) is less than 2mm thick. The hot rolling may result in a reduction in thickness of the steel strip of between 15 and 50%, and more desirably between 30 and 40%.

[0012] Illustratively, the microstructure of the cast strip has substantial fine ferrite grains having a grain size in the range of 5-20 microns, and ultrafine ferrite grains having a grain size in the range of 1-4 microns. Illustratively, the ultrafine ferrite grains comprise 30-40% of the strip.

[0013] The temperature at the mill entry can be slightly above the Ar₃ temperature, resulting in substantial transformation in the roll bite.

[0014] In order that the disclosure may be more fully explained, an example will be described with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a vertical cross-section through a steel strip casting and rolling installation which is operable in accordance with the present invention;

[0016]FIG. 2 illustrates essential components of a twin roll caster incorporated in the installation;

[0017]FIG. 3 is a vertical cross-section through part of the twin roll caster;

[0018]FIG. 4 is a cross-section through end parts of the caster.

[0019]FIG. 5 is a cross-section on the line 5-5 in FIG. 4; and

[0020]FIG. 6 is a view on the line 6-6 in FIG. 4.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] The illustrated casting and rolling installation comprises a twin roll caster denoted generally as 11 which produces a cast steel strip 12 which passes in a transit path 10 across a guide table 13 to a pinch roll stand 14. Immediately after exiting the pinch roll stand 14, the strip passes into a hot rolling mill 15 comprising roll stands 16 in which it is hot rolled to reduce its thickness under temperature. The thus rolled strip exits the rolling mill and passes to a run out table 17 on which it is subjected to cooling by means of cooling headers 18. The strip is then passed between pinch rolls 20A of a pinch roll stand 20 to a coiler 50.

[0022] Twin roll caster 11 comprises a main machine frame 21 which supports a pair of parallel casting rolls 22 having casting surfaces 22A. Molten metal is supplied during a casting operation from a ladle 23 through a refractory ladle outlet shroud 24 to a tundish 25 and thence through a metal delivery nozzle 26 into the nip 27 between the casting rolls 22. Hot metal thus delivered to the nip 27 forms a pool 30 above the nip and this pool is confined at the ends of the rolls by a pair of side closure dams or plates 28 which are applied to stepped ends of the rolls by a pair of thrusters 31 comprising hydraulic cylinder units 32 connected to side plate holders 28A. The upper surface of pool 30 (generally referred to as the “meniscus” level) may rise above the lower end of the delivery nozzle so that the lower end of the delivery nozzle is immersed within this pool.

[0023] Casting rolls 22 are water cooled so that shells solidify on the moving roll surfaces and are brought together at the nip 27 between them to produce the solidified strip 12 which is delivered downwardly from the nip between the rolls.

[0024] The twin roll caster may be of the kind which is illustrated and described in some detail in U.S. Pat. No. 5,184,668 and 5,277,243 or U.S. Pat. No. 5,488,988 and reference may be made to those patents for appropriate constructional details which form no part of the present invention.

[0025] The installation is manufactured and assembled to form a single very large scale enclosure denoted generally as 37 defining a sealed space 38 within which the steel strip 12 is confined throughout a transit path from the nip between the casting rolls to the entry nip 39 of the pinch roll stand 14.

[0026] Enclosure 37 is formed by a number of separate wall sections which fit together at various seal connections to form a continuous enclosure wall. These comprise a wall section 41 which is formed at the twin roll caster to enclose the casting rolls and a wall section 42 which extends downwardly beneath wall section 41 to engage the upper edges of scrap box 33 when the scrap box is in its operative position so that the scrap box becomes part of the enclosure. The scrap box and enclosure wall section 42 may be connected by a seal 43 formed by a ceramic fibre rope fitted into a groove in the upper edge of the scrap box and engaging flat sealing gasket 44 fitted to the lower end of wall section 42. Scrap box 33 may be mounted on a carriage 45 fitted with wheels 46 which run on rails 47 whereby the scrap box can be moved after a casting operation to a scrap discharge position. Cylinder units 40 are operable to lift the scrap box from carriage 45 when it is in the operative position so that it is pushed upwardly against the enclosure wall section 42 and compresses the seal 43. After a casting operation the cylinder units 40 are released to lower the scrap box onto carriage 45 to enable it to be moved to scrap discharge position.

[0027] Enclosure 37 further comprises a wall section 48 disposed about the guide table 13 and connected to the frame 49 of pinch roll stand 14 which includes a pair of pinch rolls 14A against which the enclosure is sealed by sliding seals 60. Accordingly, the strip exits the enclosure 38 by passing between the pair of pinch rolls 14A and it passes immediately into the hot rolling mill 15. The spacing between pinch rolls 50 and the entry to the rolling mill should be as small as possible and generally of the order of 5 metres or less so as to control the formation of scale prior to entry into the rolling mill.

[0028] Most of the enclosure wall sections may be lined with fire brick and the scrap box 33 may be lined either with fire brick or with a castable refractory lining.

[0029] The enclosure wall section 41 which surrounds the casting rolls is formed with side plates 51 provided with notches 52 shaped to snugly receive the side dam plate holders 28A when the side dam plates 28 are pressed against the ends of the rolls by the cylinder units 32. The interfaces between the side plate holders 28A and the enclosure side wall sections 51 are sealed by sliding seals 53 to maintain sealing of the enclosure. Seals 53 may be formed of ceramic fibre rope.

[0030] The cylinder units 32 extend outwardly through the enclosure wall section 41 and at these locations the enclosure is sealed by sealing plates 54 fitted to the cylinder units so as to engage with the enclosure wall section 41 when the cylinder units are actuated to press the side plates against the ends of the rolls. Thrusters 31 also move refractory slides 55 which are moved by the actuation of the cylinder units 32 to close slots 56 in the top of the enclosure through which the side plates are initially inserted into the enclosure and into the holders 28A for application to the rolls. The top of the enclosure is closed by the tundish, the side plate holders 28A and the slides 55 when the cylinder units are actuated to apply the side dam plates against the rolls. In this way the complete enclosure 37 is sealed prior to a casting operation to establish the sealed space 38 whereby to limit the supply of oxygen to the strip 12 as it passes from the casting rolls to the pinch roll stand 14. Initially the strip will take up all of the oxygen from the enclosure space 38 to form heavy scale on the strip. However, the sealing of space 38 controls the ingress of oxygen containing atmosphere below the amount of oxygen that could be taken up by the strip. Thus, after an initial start up period the oxygen content in the enclosure space 38 will remain depleted so limiting the availability of oxygen for oxidation of the strip. In this way, the formation of scale is controlled without the need to continuously feed a reducing or non-oxidising gas into the enclosure space 38. In order to avoid the heavy scaling during the start-up period, the enclosure space can be purged immediately prior to the commencement of casting so as to reduce the initial oxygen level within the enclosure and so reduce the time for the oxygen level to be stabilised as a result of the interaction of oxygen from the scaled enclosure due to oxidation of the strip passing through it. The enclosure may conveniently be purged with nitrogen gas. It has been found that reduction of the initial oxygen content to levels of between 5% to 10% will limit the sealing of the strip at the exit from the enclosure to about 10 microns to 17 microns even during the initial start-up phase.

[0031] In a typical caster installation the temperature of the strip passing from the caster will be of the order of 1400° C. and the temperature of the strip presented to the mill may be about 900-110° C. The strip may have a width in the range 0.9 m to 1.8 m and a thickness in the range 0.7 mm to 2.0 mm. The strip speed may be of the order of 1.0 m/s. It has been found that with strip produced under these conditions it is quite possible to control the leakage of air into the enclosure space 38 to such a degree as to limit the growth of scale on the strip to a thickness of less than 5 microns at the exit from the enclosure space 38, which equates to an average oxygen level of 2% with that enclosure space. The volume of the enclosure space 38 is not particularly critical since all of the oxygen will rapidly be taken up by the strip during the initial start up phase of a casting operation and the subsequent formation of scale is determined solely by the rate of leakage of atmosphere into the enclosure space though the seals. It is preferred to control this leakage rate so that the thickness of the scale at the mill entry is in the range 1 micron to 5 microns. Experimental work has shown that the strip needs some scale on its surface to prevent welding and sticking during hot rolling. Specifically, this work suggests that a minimum thickness of the order of 0.5 to 1 micron is necessary to ensure satisfactory rolling. An upper limit of about 8 microns and preferably 5 microns is desirable to avoid “rolled-in scale” defects in the strip surface after rolling and to ensure that scale thickness on the final product is no greater than on conventionally hot rolled strip.

[0032] Illustratively, hot rolling of the strip in the hot rolling mill 15 is controlled to achieve a thickness reduction of 30-40% within a strip temperature range at the entry to the mill centered around the Ar₃ temperature resulting in substantive transformation in the roll bite.

[0033] Hot rolling continuously cast strip under these hot rolling temperature and reduction conditions produces a microstructure that is a mixture of ultrafine polygonal ferrite grains and fine polygonal ferrite grains that provides the strip with superior mechanical properties including a yield strength of at least 400 MPa and a total elongation of at least 30%.

[0034] Experimental work is the basis of the present disclosure. The experimental work involved hot rolling silicon/manganese killed low carbon steel to achieve thickness reductions of 30-40% starting at a temperature of 860° C. and passing through the Ar₃ temperature of the steel under strain. It was found that these hot rolling conditions produced a very fine microstructure in the cast rolled strip. Specifically, approximately 30% of the thickness near the surfaces of the strip cast under these hot rolling conditions consisted of ultrafine equiaxed ferrite grains of 1-4 microns and the remainder of the strip had a very fine polygonal equiaxed ferrite microstructure of 5-20 microns. The microstructure obtained provided superior mechanical properties of yield strength of 400 MPa and total elongation of greater than 30%. The above type of microstructure was observed only when the strip was hot rolled under the described hot rolling temperature and reduction conditions. These observations suggest that strain-induced transformation of austenite to ferrite played a role in the observed grain refinement. It will be appreciated that the transformation under strain takes place within a temperature range such that acceptable microstructures may be obtained in cases where the starting temperature is above or below the Ar₃ temperature of the steel.

[0035] Although the invention has been illustrated and described in detail in the foregoing drawings and description with reference to several embodiments, it should be understood that the description is illustrative and not restrictive in character, and that the invention is not limited to the disclosed embodiments. Rather, the present invention covers all variations, modifications and equivalent structures that come within the scope and spirit of the invention. Additional features of the invention will become apparent to those skilled in the art upon consideration of the detailed description, which exemplifies the best mode of carrying out the invention as presently perceived. Many modifications may be made to the present invention as described above without departing from the spirit and scope of the invention. 

The claims defining the invention are as follows:
 1. A method of producing steel strip having a yield strength of at least 400 MPa and a total elongation of at least 30% which includes the steps of: (a) continuously casting molten low carbon steel into a strip; (b) hot rolling the cast strip within a temperature range such that the strip passes through the austenite to ferrite transformation under strain during rolling to produce a strip having a yield strength above 400 MPa and an elongation in excess of 30%.
 2. The method of claim l, wherein the temperature range begins above and passes through the Ar₃ transformation temperature.
 3. The method of claim 1, wherein the temperature range begins below the Ar₃ transformation temperature.
 4. The method of claim 1, wherein the steel strip cast in step (a) is less than 2 mm thick.
 5. The method of claim 1, wherein the hot rolling achieves a reduction in thickness of the steel strip of between 15 and 50%.
 6. The method of claim 1, wherein the hot rolling achieves a reduction in thickness of the steel strip of between 30 and 40%.
 7. A cast steel strip having a yield strength of at least 400 MPa and a total elongation of at least 30% made by the following steps: (a) continuously casting molten low carbon steel into a strip; (b) hot rolling the cast strip within a temperature range such that the strip passes through the austenite to ferrite transformation under strain during rolling to produce a strip with a microstructure providing a strip with strength above 400 MPa and an elongation in excess of 30%.
 8. The cast steel strip of claim 7, wherein the temperature range begins above and passes through the Ar₃ transformation temperature.
 9. The cast steel strip of claim 7, wherein the temperature range begins below the Ar₃ transformation temperature.
 10. The cast steel strip as described in claim 7 wherein the microstructure of the cast strip has substantial fine ferrite grains having a grain size in the range of 5-20 microns.
 11. The cast steel strip as described in claim 7 wherein the microstructure of the cast strip has substantial ultrafine ferrite grains having a grain size in the range of 1-4 microns.
 12. The cast steel strip as described in claim 11 wherein the ultrafine ferrite grains comprise 30-40% of the strip. 